The early tectono-magmatic evolution of the southern province: implications from the Agnew Intrusion, central Ontario, Canada

The Palaeoproterozoic Southern Province comprises a thick, continental rift related volcanic-sedimentary sequence along the southern margin of the Arc haean Superior Province. The Agnew Intrusion (50 km(2)), which is a member of the East Bull Lake suite of layered intrusions, occurs adjacent to the S uperior Province - Southern Province boundary in central Ontario, Canada, a nd provides an opportunity to examine the early tectono-magmatic evolution of a Palaeoproterozoic rifting event. The Agnew Intrusion is a well-exposed , 2100 m thick, layered gabbronoritic to leucogabbronoritic pluton. It was the product of at least four recognizable, but chemically similar, high-Al2 O3 and low-TiO2 magma pulses. Structural data, coupled with excellent strat igraphic correlations between the Agnew Intrusion and other East Bull Lake suite layered intrusions, suggest that these plutons are erosional remnants of one or more sill-like bodies that may originally have formed an extensi ve, subhorizontal mafic sheet. We argue on the basis of field evidence that the early evolution of the Southern Province was characterized by a large, mantle plume induced magmatic event that gave rise to a Palaeoproterozoic continental flood basalt province. However, the incompatible trace element characteristics of the Agnew Intrusion parental magma (i.e., large ion lith ophile and light rare earth element enrichment and high field strength elem ent depletion) are more typical of modern subduction-modified subcontinenta l lithospheric mantle. Given that this is a prevailing geochemical signatur e of mafic rocks in the Archaean-Palaeoproterozoic, we suggest that there w as a fundamental difference in both the composition and structure between t he ancient and more modern mantle. "Subduction-like" geochemical signatures may have been imparted to the entire developing mantle during early Earth differentiation.